Vehicle drive-trains known from the prior art are usually formed in each case by an internal combustion engine and at least one transmission system arranged between the internal combustion engine and a drive output, by means of which gear ratios called for in each case depending on the operating situation can be obtained.
Such transmission systems, which are known per se and are made as automatic transmissions, are constructed with shifting elements for obtaining the various gear ratios, by means of the shifting elements, an applied torque can be transmitted by friction. When the engagement of a defined gear is called for, in each case at least one or more of the frictional shifting elements are disengaged from the force flow of a transmission while at least one or more other frictional shifting elements are engaged in the force flow of the transmission in order to obtain the gear required. During the engagement process of a frictional shifting element, no special synchronization measures are needed to ensure a desired level of shifting comfort, since a desired shifting comfort can be achieved with frictional shifting elements at defined contact pressures within a broad range of rotation speed differences.
As is known per se, since owing to drag losses that occur in the area of open frictional shifting elements such transmission systems can only be operated with inadequate efficiencies, certain frictional shifting elements are replaced by interlocking shifting elements. In automatic transmissions constructed with both frictional shifting elements and with at least one interlocking shifting element and with a corresponding gearset design, to implement a defined shift command at least one interlocking shifting element has to be engaged in a force flow of a transmission.
Disadvantageously, compared with frictional shifting elements, interlocking shifting elements with or without additionally designed synchronization means can only be shifted comfortably when their speed differences are very small, i.e. close to their synchronous speed, and accordingly the operation of a transmission constructed with at least one interlocking shifting element and designed as an automatic transmission is made undesirably more difficult, at least when carrying out some shifts in which an interlocking shifting element is involved. In some circumstances a shift called for can only be completed after the passage of an undesirably long shifting time, since before the interlocking shifting element involved in the required shift can be closed, the speed difference in the area of the interlocking shifting element has to be adjusted to a value at which the interlocking shifting element can be changed to its closed operating condition.
However, additional design measures for synchronizing interlocking shifting elements of transmission systems in order to carry out a required shift within an acceptable shifting time along with a high level of shifting comfort entail an undesirable increase both of production costs and of the fitting space occupied by the transmission.
From EP 00992706 A2 a vehicle transmission is known, which comprises both frictional and interlocking shifting elements. A speed difference between the shifting element halves of the interlocking shifting element is in this case reduced by changing the engine speed, to a point where the interlocking shifting element can be engaged. Disadvantageously, in this geared change-speed transmission no shifting element must be closed in the neutral condition, so that on shifting from neutral into a gear an interlocking shifting element must first be engaged, with prior adaptation of the engine speed, which is time-consuming.